Experimental animals (monauralized guinea pigs and chinchillas) will be trained by operant conditioning procedures with food reinforcement on one or more of several protocols for hearing evaluation. In addition to pure-tone audiometry, measures of differential thresholds for frequency and intensity, psychophysical turning curves, and threshold functions in high-pass masking noise will be determined. When normal behavioral baselines are stable following training, the normal inner ear of each animal will be lesioned by exposure to low-frequency noise, application of a cryoprobe to the bony wall of the cochlea, or surgical destruction of a portion of the organ of Corti. These treatments all result in damage to the apical-most portion of the organ of Corti that is primarily responsible for detection and discrimination of low- frequency sounds. Following these lesioning procedures, hearing assessment by behavioral means will be continued until sufficient data has been obtained on changes in the measures of hearing and until no further changes are noted for a least 30 days. Animals will then be sacrificed and their temporal bones taken for microdissection and analysis of th cochlea by light and electron microscopy. It is hoped that the relation between the pathology observed in the cochlea and the changes in the bahavioral measures will increase our understanding of the function of the apical region of the cochlea in hearing. These studies are particularly important in understanding the apparent ability of the auditory system to compensate for damage to the apex of the cochlea. This compensation can occur to such an extent that, even with total loss of apical receptors, hearing impairment for low frequencies is slight relative to the profound deafness that results from total hair-cell loss in the base. On goal of this research is a thorough description of the role played by higher-frequency receptors in detection of low frequencies under conditions of total loss of apical hair cells. The studies are also important in view of demonstrations that partial damage can exist in apical regions that is not detected by routine audiometric threshold tests. Developing test that could better detect such damage is an additional goal of this research.